The present invention relates to storage racks for spent nuclear fuel assemblies, and more particularly to an improved rack structure usable for either temporary or permanent storage of spent fuel.
Nuclear reactors consist of an array of fuel rods containing the nuclear fuel. The fuel rods are metal tubes, typically from 8 to 15 feet in length and about 1/2 inch in diameter, and are supported in groups in fuel assemblies which may comprise a considerable number of rods. The large reactors utilized for power generation contain a large number of these fuel assemblies arranged in a suitable configuration.
After an extended period of operation, the irradiated or spent fuel assemblies must be removed from the reactor and replaced. The spent fuel rods contain residual amounts of the original fuel material, and varying amounts of numerous fission products resulting from fission of the nuclei of the original fuel and other nuclear reactions, as well as from radioactive decay of initially formed fission products. Certain of these materials are themselves fissionable. Many of the fission products are highly radioactive, at least initially, and thus produce considerable heat, while the entire fuel assembly is dangerously radioactive. The fuel rods can be reprocessed by chemically separating the fissionable material for reuse as fuel and recovering various other fission products, such as certain rare earth elements, for example, which have substantial commercial value.
Suitable facilities must be provided for the storage of these highly radioactive assemblies after removal from the reactor until they can be reprocessed, or if they are not to be reprocessed, then permanent storage must be provided which will insure safe disposition of these fuel assemblies for an indefinite period of time. Such storage, either temporary or permanent, presents serious problems since the fuel assemblies are initially highly radioactive and generate a great deal of heat. They must, therefore, be kept submerged in water which serves as a coolant to prevent overheating as well as a radiation shield and moderator for the fast neutrons which are still being emitted. It is also necessary to be sure that the assemblies are stored in a manner that will prevent criticality. After some period of time, the heat generation within the fuel assemblies declines, since many of the initial fission products have relatively short half-lives, and the nature of the storage problem changes as less heat is generated and the radiation hazard decreases.